BACKGROUND Major depression is one of the main mental illnesses in old age, with acute exacerbated episodes requiring treatment in geriatric psychiatry. A meta-analysis showed that aerobic exercise in moderate intensity has large effects in older adults with major depression, but there is no evidence of aerobic exercise in geriatric psychiatry. Therefore, this study aims to analyze the feasibility and effects of an ergometer-based aerobic exercise on depressive symptoms. METHODS A single-center randomized controlled trial will be conducted in an acute geriatric psychiatric hospital. Inpatients allocated to the intervention group will receive a 2-week aerobic ergometer program. The control group will receive seated flexibility exercise in addition to usual care. The overall effects on the patients' depressive symptoms will be measured by clinical global impression of change (CGI) as the primary outcome. Changes in depressive symptom domains, physical (in)activity, and aerobic performance as well as the dosage of applied antidepressants will be examined as secondary outcomes. CONCLUSIONS This short-term aerobic exercise program is expected to decrease depressive symptoms in acute exacerbated periods in older adults. The results may increase the evidence for implementing physical activity interventions in acute hospital settings. The disease-related motivation for exercise in acute exacerbated depressive periods will be the most challenging aspect. The treatment of depression requires new cost-effective approaches, especially in acute geriatric psychiatry with potential benefits for patients, family members, and clinicians. BACKGROUND German Clinical Trial Register ID: DRKS00026117 TRIAL STATUS: Protocol Version 1.2 dated February 23, 2022. By February 23, 2022, the trial had recruited a total of 15 participants in two wards at the Department of Geriatric Psychiatry at the LVR-Hospital Cologne. Recruitment started on November 12, 2021. The recruitment is expected to continue for at least 12 months.